A shock absorber includes a cylinder, a piston rod inserted into the cylinder to be free to move, and a piston that is attached to an outer periphery of the piston rod and inserted into the cylinder to be free to slide, and divides an interior of the cylinder into an expansion side chamber and a contraction side chamber. The shock absorber generates damping force by applying resistance to a flow of working oil that passes between the expansion side chamber and the contraction side chamber in accordance with a valve laminated onto the piston.
In a shock absorber incorporated into a suspension of a vehicle, vibration in a resonance frequency band of a vehicle body and an axle is damped by generating damping force in response to this vibration, and as a result, passenger comfort in the vehicle can be improved. However, when excessively large damping force is generated in response to vibration in a frequency band other than the aforesaid resonance frequency band, a vibration suppression effect of a suspension spring, a rubber cushion, and so on may be impaired, and as a result, the passenger comfort may deteriorate.
JP2003-42214A discloses a shock absorber having a valve in which an orifice and a leaf valve are provided in parallel. When a piston speed is in a low speed region, the shock absorber generates damping force mainly using the orifice, and when the piston speed is in a high speed region, the leaf valve opens such that the shock absorber generates damping force mainly using the leaf valve.
As regards a damping characteristic (a characteristic of the damping force relative to the piston speed) of the shock absorber, as shown in FIG. 13, a characteristic particular to the orifice, in which the damping force is proportionate to the square of the piston speed, is obtained when the piston speed is in the low speed region, and a characteristic particular to the leaf valve, in which the damping force is proportionate to the piston speed, is obtained when the piston speed is in the high speed region. Hence, by using the orifice when the piston speed is in the low speed region, the damping force can be prevented from becoming too large, and by using the leaf valve when the piston speed is in the high speed region, the damping force can be prevented from becoming too small. Accordingly, a pseudo-damping characteristic that corresponds to the vibration frequency can be realized, and as a result, the passenger comfort of the vehicle can be improved.